1. Field of Invention
This invention is in the field of enhancing Synthetic Aperture Radar (SAR) images at squint angles approaching the direction of travel of the SAR platform using search (swath) mode.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses. These pulses are from a relatively small antenna on a moving platform. As the platform moves, the information contained in the pulses is coherently combined to arrive at a high resolution SAR image.
The plurality of returns creating a SAR image generated by the transmitted pulses along a presumed known path of the platform makes up an array. Theoretically, during the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.
SAR imaging can be divided into two general types, spot SAR and search (swath) SAR. In spot SAR, the antenna is pointed towards a point, or spot, around which the image is rendered. In contrast, for swath SAR, the antenna is held at a constant angle, the squint angle, with respect to the travel direction of the platform, thus yielding a strip image as the platform moves forward. Spotlight mode produces two dimensional images of limited size of a limited area by steering the antenna beam to the center of the map (image) center for the duration of an array. Conversely, search mode produces image strips of theoretically unlimited length by maintaining a fixed azimuth antenna orientation during a SAR data collection period, or array. Unlike spotlight mode that typically produces images oriented in the range-azimuth direction, swath SAR produces images oriented in the along track and cross track direction.
The are substantial SAR search (swath) mode limitations. For example, uncompensated platform motion during search mode results in image smearing in the azimuth direction caused by pulse data that affects azimuth response. Because target image in search SAR smears in the direction of both image axes, it is difficult to estimate and correct residual phase error for autofocus using one dimensional batch processing.
Another difficulty presented during search mode is in merging the phase error for multiple parts forming the SAR image of a patch. The boundaries where parts of the patch meet are blurred, not well defined as the phase definitions in that area are not clearly defined, unlikely to be continuous. Hence, phase errors need to be compensated for and cannot be estimated for all ground targets in a continuous manner. These individual parts of a patch need to be corrected for a continuous image. Lack of such corrections will introduce unwanted, spurious high frequency components in the resulting image thereby degrading its quality and utility.
To add to above limitations of search mode SAR, as the swath being imaged becomes wider and ahead of the moving platform, forcing a high squint angle departing substantially from the broadside direction, a number of radar returns in the area of the high squint angle are no longer usable. Thus, at high squint angles image quality, resolution, signal to noise ratio, and false alarm rates are negatively impacted.